Duplicate notification when a requisition is filled

Summary:

I have a requirement to send the alert IRC_Requisition_Filled to a shared mailbox. To test, I added the Mail communication method and an email address. I removed the other recipients as well. This worked, as the notification was sent successfully. However, when I try to add another email, the notification was sent twice. Even when I added a worklist (HM), and just left the email address, I still got duplicate notification.

My expectation is that the notification will only be sent once to each recipient which is not happening during my testing.

I also tried to set Stop Duplicate Messages to 'Yes' but notifications are still duplicated.